Electronics, microelectronics and microelectromechanics require as starting materials semiconductor wafers with extreme requirements made of global and local flatness, single-side-referenced flatness (nanotopology), roughness and cleanness. Semiconductor wafers are wafers composed of semiconductor materials such as elemental semiconductors (silicon, germanium), compound semiconductors (for example composed of an element of the third main group of the periodic table such as aluminum, gallium or indium and an element of the fifth main group of the periodic table such as nitrogen, phosphorus or arsenic) or the compounds thereof (for example Si1-xGex, 0<x<1).
In accordance with the prior art, semiconductor wafers are produced by means of a multiplicity of successive process steps which can generally be classified into the following groups:
(a) producing a usually monocrystalline semiconductor rod;
(b) slicing the rod into individual wafers;
(c) mechanical processing;
(d) chemical processing;
(e) chemomechanical processing;
(f) if appropriate additional production of layer structures.
A method designated “planetary pad grinding” (“PPG”, pad grinding with planetary kinematics) is known as a particularly advantageous method from the group of mechanical processing steps. The method is described for example in DE102007013058A1, and an apparatus suitable therefor is described for example in DE19937784A1. PPG is a method for the simultaneous double-side grinding of a plurality of semiconductor wafers, wherein each semiconductor wafer lies such that it is freely movable in a cutout in one of a plurality of running disks (insert carriers) caused to rotate by means of a rolling apparatus and is thereby moved on a cycloidal trajectory. The semiconductor wafers are processed in material-removing fashion between two rotating working disks. Each working disk comprises a working layer containing bonded abrasive. The working layers are present in the form of structured grinding pads which are fixed on the working disks adhesively, magnetically, in a positively locking manner (for example hook and loop fastener) or by means of vacuum.
A similar method is so-called “flat honing” or “fine grinding”. In this case, a plurality of semiconductor wafers in the arrangement described above for PPG are guided on the characteristic cycloidal paths between two large rotating working disks by means of a rolling apparatus. Abrasive grain is fixedly bonded into the working disks, such that the material removal is effected by means of grinding. In the case of flat honing, the abrasive grain can be bonded directly into the surface of the working disk or be present in the form of an areal covering of the working disk by means of a multiplicity of individual abrasive bodies, so-called “pellets”, which are mounted onto the working disk (P. Beyer et al., Industry Diamanten Rundschau IDR 39 (2005) III, page 202).
In the case of PPG and pellets grinding, the working disks are embodied in ring-shaped fashion, and the rolling apparatus for the running disks is formed from an inner and an outer pin wheel, which are arranged concentrically with respect to the rotation axis of the working disks. Inner and outer pin wheels thus form sun gear and internal gear of a planetary gear arrangement by means of which the running disks revolve with inherent rotation like planets around the central axis of the arrangement—hence the name “running disks”.
Finally, a further method similar to PPG grinding is simultaneous double-side orbital grinding, which is described for example in US 2009/0311863A1. In the case of orbital grinding, too, the semiconductor wafers are inserted in receiving openings of an insert carrier, which guides them during processing between the rotating working disks. In contrast to PPG or pellets grinding, however, an orbital grinding apparatus has only a single insert carrier, which covers the entire working disk. The working disks are not embodied in ring-shaped fashion, but rather in circular fashion. The insert carrier is guided by means of a plurality of guide rollers arranged outside the working disk and around the circumference thereof. The rotary spindles of said guide rollers are eccentrically connected to drive spindles. As a result of the rotation of said drive spindles, the guide rollers perform an eccentric movement and thereby drive a gyroscopic or orbital movement of the insert carrier. In the case of orbital grinding, therefore, the insert carrier does not rotate about its own central axis, nor does it revolve about the rotation axis of the working disks, but rather performs an oscillating movement in the form of small circles over the area of the working disks. This orbital movement is characterized by the fact that, under each semiconductor wafer thus guided by the insert carrier, there is always a respective area in the spatially fixed reference system which, during the movement, lies continuously completely within the area swept over by the semiconductor wafer.
DE102007049811A1 stipulates that, for carrying out the PPG or pellets grinding method, use is made of running disks whose thickness is equal to or thinner than the final thickness of the semiconductor wafers processed thereby. This also applies to orbital grinding, for the same reasons. The running disks (PPG, pellets grinding) and the insert carrier (orbital grinding) are therefore very thin, for example less than typically 0.8 mm when processing a silicon wafer having a diameter of 300 mm. Furthermore, DE102007049811A1 stipulates that the running disks and the insert carrier have to be sufficiently stiff in order to withstand the forces acting during processing, and that their surfaces which come into contact with the working layer during processing have to be particularly resistant to wear and are permitted to have only little interaction with the working layer, in order that the working layer does not become blunt and need to be reconditioned (redressed) through undesirably frequent and complex trimming. In accordance with DE102007049811A1, therefore, running disks suitable for carrying out the PPG method, for example, preferably comprise a core composed of a first material, which has a high stiffness, said core being completely or partly coated with a second material, and also at least one opening for receiving a semiconductor wafer. Preferably, in accordance with DE102007049811A1, a thermosetting polyurethane having a hardness of between Shore 40 A and Shore 80 A is used as second material. This has proved to be particularly resistant to wear in relation to diamond, the abrasive substance preferably used.
In this case, the antiwear layer is applied by spraying, dipping, flooding, spreading, rolling or blade coating. However, preference is typically given to coating by molding in an injection mold, into which the first material is inserted in a centered manner with space for the coating on the front and rear sides. Alternatively, coating with a layer with excess thickness and subsequent grinding back to the desired target thickness are also known.
DE102007049811A1 explains that very high frictional forces act on the antiwear layers known in the prior art. Said forces are much greater than the frictional forces owing to the chipping capacity exerted by the material removal on the semiconductor wafer.
On account of the high forces, the stiffness-imparting core of the running disk has to be very thick in order that the running disk is still sufficiently stable. As a result, only a small proportion of thickness—a maximum of 100 μm but in practice significantly less—remains for the coating of the running disk, which considerably restricts the service life thereof and means high costs for the wearing part running disk.
Moreover, the high frictional forces have the effect that the semiconductor wafers, during processing, are not moved in a manner which is as far as possible with low forces and “free floating”, as desired. As a result, the advantages of simultaneous double-side processing which lead to a particularly high flatness of the semiconductor wafer are partly nullified if the processing is carried out using running disks known in the prior art.
According to DE102007049811A1, the high frictional forces owing to the small layer thickness bring about particularly harmful peeling forces between core material and coating of the insert carrier. Said forces lead to premature detachment of the coating through delamination to an increased extent. In order to counteract layer detachment, which leads to the fracture of the semiconductor wafer and usually also of the running disk, WO2008/064158A2, for example, describes the use of an additional layer of an adhesion promoter between core material and antiwear coating of the running disk. However, this, too, does not solve the problem of the excessively low layer adhesion, such that antiwear-coated running disks known in the prior art are unsuitable for carrying out the PPG method and related grinding methods.
Finally, DE102007049811A1 and WO2008/064158A1 also describe running disks, the core material of which is only partly coated with an antiwear layer. However, these prove to be particularly susceptible to premature layer detachment and are therefore likewise unsuitable for the processing of semiconductor wafers.